I. Field of the Invention
The present invention pertains generally to the field of wireless communications, and more specifically to providing an efficient method and apparatus for transmitting time-sensitive information in a wireless voice-over-data communication system.
II. Background
The field of wireless communications has many applications including cordless telephones, paging, wireless local loops, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. (As used herein, the term “cellular” systems encompasses both cellular and PCS frequencies.) Various over-the-air interfaces have been developed for such cellular telephone systems including frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, such as IS-95A, IS-95B (often referred to collectively as IS-95), ANSI J-STD-008, IS-99, IS-657, IS-707, and others, are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. An exemplary cellular telephone system configured substantially in accordance with the use of the IS-95 standard is described in U.S. Pat. No. 5,103,459 entitled “System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System”, which is assigned to the assignee of the present invention and incorporated herein by reference. The aforesaid patent illustrates transmit, or forward-link, signal processing in a CDMA base station. Exemplary receive, or reverse-link, signal processing in a CDMA base station is described in U.S. application Ser. No. 08/987,172, filed Dec. 9, 1997, entitled MULTICHANNEL DEMODULATOR, which is assigned to the assignee of the present invention and incorporated herein by reference. In CDMA systems, over-the-air power control is a vital issue. An exemplary method of power control in a CDMA system is described in U.S. Pat. No. 5,056,109 entitled “Method and Apparatus for Controlling Transmission Power in A CDMA Cellular Mobile Telephone System” which is assigned to the assignee of the present invention and incorporated herein by reference.
Under Interim Standards IS-99 and IS-657 (referred to hereinafter collectively as IS-707), an IS-95-compliant communications system can provide both voice and data communications services. Data communications services allow digital data to be exchanged between a transmitter and one or more receivers over a wireless interface. Examples of the type of digital data typically transmitted using the IS-707 standard include computer files and electronic mail. More recently, data protocols, such as those specified by IS-707, have been used to transmit time-sensitive information, such as audio or video information. Transmitting time-sensitive information using data protocols is advantageous in such applications as encryption and distributing the audio information to a number of computers via a large computer network, such as the Internet.
In accordance with the IS-707 standard, which includes IS-707.4 for asynchronous data services and IS-707.5 for packet data services, the data exchanged between a transmitter and a receiver is processed in discrete packets, otherwise known as data packets or data frames, or simply frames. To increase the likelihood that a frame will be successfully transmitted during a data transmission, IS-707 employs a radio link protocol (RLP) to track the frames transmitted successfully and to perform frame retransmission when a frame is not transmitted successfully. In addition to RLP, higher layer data protocols are also used to ensure that frames are successfully received. For example, the well known TCP protocol is used in an asynchronous data service environment, (as explained in IS-707.4) and the UDP protocol is used in a packet data service environment (as explained in IS-707.5) in addition to the RLP protocol.
One of the primary problems of transmitting time-sensitive information using data protocols is the delay, or latency, caused by the non-continuous nature of data communications. Delays of more than a few hundred milliseconds can result in disruptions in the time-sensitive information being transmitted. When transmitting data, such as computer files, time delays are easily tolerated due to the non real-time nature of data. However, time-sensitive information needs to received continuously in order to be faithfully recreated.
At a transmitter, the TCP and UDP protocols mentioned above rely on filling a TCP or UDP segment with a selected number of bits, often referred to as a minimum segment size, then transmitting the segment. If a large amount of data is available for transmission in a voice-over-data system, a delay may be introduced in converting the available time-sensitive information into numerous TCP or UDP segments. On the other hand, if a small amount of time-sensitive information is available to be transmitted, a TCP or UDP segment will not be immediately generated unless the amount of available time-sensitive information is enough to fill the segment. This also introduces a time delay into the transmission process.
If the TCP or UDP minimum segment size is chosen to be relatively large, latency will increase due to the fact that a segment will only be transmitted if the minimum segment size is met. On the other hand, if the minimum segment size is chosen to be relatively small, a significant drain on processing resources will occur in the transmitter to process the numerous segments needed for transmission of even a small amount of information.
What is needed is a method and apparatus for optimizing the segment size so that time-sensitive information is transmitted smoothly, without a change to existing data protocols.